The present invention relates to an image forming apparatus that forms a toner image on a transfer sheet for outputting, as a copying machine, a printer, a facsimile machine and a multifunctional machine including the aforesaid equipment, and in particular, to control for correcting a transfer attitude of a transfer sheet that is conveyed to a position where a toner image is transferred onto the transfer sheet, wherein the transfer attitude is an attitude to receive a toner image.
In the case of recent image forming apparatuses each being connected with a personal computer, those wherein images and information composed of images and character information (which is also called image information) are inputted, then, printed and outputted are more in terms of a number than those wherein pieces of information composed of characters, figures or of symbols (which is also called character information) are inputted, then, subjected to image forming and outputted.
In the case of outputting image information such as color photos, for example, there are many occasions of the structure wherein an image that is cut off square is fitted in a printable area per page of a transfer sheet. Therefore, if a transfer sheet is conveyed and printed under the condition that the transfer sheet is inclined to the conveyance direction for the transfer sheet (hereinafter referred to as “skew”), a blank space provided between an edge portion of the transfer sheet is inclined because a boundary portion of the square image is in a form of a straight line, and “transfer sheet skew” is more emphasized, resulting in a problem that the transfer sheet skew is interpreted as “image skew” depending on a way of looking at.
Therefore, when outputting image information, the transfer sheet skew is more conspicuous than in the case of outputting character information, thus, it has become a demand that even a slight skew of a transfer sheet which has been allowed so far may also be corrected for conducting forming of high grade images.
In particular, in an image forming apparatus of a type to form a color image by transferring toner images each being formed by each of Y (yellow), M (magenta), C (cyan) and K (black) color toners, and by superposing them, for example, if the skew of a transfer sheet as one mentioned above is caused, a phenomenon called “out of color registration” is generated, and images having lower grade are formed, which has been a problem.
Further, if a transfer sheet deviates in a transverse direction from a transfer standard position, there may be a problem that a transfer position of an image is different for each transfer sheet so that an appearance of the document becomes deteriorated.
Therefore, for correcting the transfer sheet skew of the above kind, there is disclosed a technology capable of correcting the transfer sheet skew which is higher in terms of accuracy than the well-known registration apparatus of a type to correct transfer sheet skew by causing a transfer sheet to hit a pair of rollers.
For example, there is disclosed a technology wherein, when an inclination detection means and a position detection means are arranged at the downstream side of the paired conveyance rollers of the registration apparatus, since detections for a skew of a transfer sheet and for a position of a side edge of the transfer sheet are made under the condition that a recording sheet (transfer sheet) is interposed by the paired conveyance rollers and if the paired conveyance rollers are rotated corresponding to the skew angle showing the skew of the transfer sheet calculated based on the results of the detections, the skew of the transfer sheet can be corrected accurately (for example, see Patent Document 1).
Further, a detection means that detects a skew from the transfer sheet conveyance direction under the condition that sheet S (transfer sheet) is interposed is provided, and when the transfer sheet is skewed, a leading edge position after t sec. after the skew is corrected by swing of a skew correction means is calculated and estimated based on detection signals from a detection means of the skew, and the conveyance speed of the paired registration rollers is calculated depending on an amount of deviation of the leading edge of the transfer sheet after t sec. thus obtained.
Then, based on the results of the calculation, the skew of the transfer sheet is corrected by swinging the skew correction means, so that a leading edge of the transfer sheet and the forefront of an image on the photoreceptor drum may be aligned at the transfer section, and the transfer sheet conveyance speed by the paired registration rollers is adjusted to the conveyance speed obtained by the moment when a leading edge of the transfer sheet arrives at the transfer section, and the transfer sheet is conveyed.
There has been disclosed a technology wherein a skew of the transfer sheet can be corrected extremely accurately without stopping the transfer sheet momentarily, by conveying the transfer sheet to the transfer section at the same speed as the circumferential speed of the photoreceptor drum, after a leading edge of the transfer sheet arrives at the transfer section (for example, see Patent Document 2).
Further, there has been published a technique that a position detecting sensor is arranged in a direction perpendicular to a transfer sheet conveying direction so as to detect an inclination of a transfer sheet by detecting a position on a side edge portion of a transfer sheet along the transfer sheet conveying direction (for example, see Patent Document 3).
(Patent Document 1) TOKUKAIHEI No. 10-67448
(Patent Document 2) TOKUKAI No. 2005-53646
(Patent Document 3) TOKUKAI No. 2005-35709
However, in the Patent Document 1, a left end position or a right end position on a leading edge of the transfer sheet that is in the direction perpendicular to the conveyance direction is not always corrected to the regular position, depending on a position of rotation center of the paired conveyance rollers, although a skew in the direction perpendicular to the conveyance direction is corrected by the rotation of the paired conveyance rollers. Therefore, it is necessary to correct a left end position or a right end position on a leading edge of the transfer sheet to the regular position by moving the transfer sheet in parallel with the direction perpendicular to the conveyance direction. Therefore, when correcting the skew by this parallel movement, the transfer sheet is sometimes skewed again, according to circumstances, which has been a problem.
In the Patent Document 2, a skew correction means is swung based on detection signals coming from a detection means, and a position of the leading edge of the transfer sheet after t sec. from the correction of the skew is estimated, for the correction. Therefore, the timing between the estimation and the actual conveyance speed of the transfer sheet, for example, is not adjusted when occasion demands, resulting in a problem that the skew cannot be corrected sufficiently, although there is an advantage to correct without stopping the transfer sheet momentarily.
In other words, there has been a problem that a skew generated after skew correction and a slight skew that failed to be corrected, for example, cannot be corrected in both the Patent Document 1 and the Patent Document 2, because the skew is corrected based on signals detected once.
Further, although Patent Document 3 discloses a correcting device to correct an inclination of a transfer sheet while sandwiching the transfer sheet, it is necessary to release sandwiching by conveying rollers to convey the transfer sheet during the inclination correction. Accordingly, there is a troublesomeness that the sandwiching operation by the correcting device and the sandwiching operation by the conveying rollers are controlled with timing.